Most oil and gas wells are drilled by using drill pipe to form the borehole. When a desired depth is reach, the operator connects together a string of casing, lowers it into the well, and cements it into the well. The operator may then drill deeper and run additional strings of casing.
In another technique, known as casing-while-drilling, the casing serves as the drill string, thus is located in the borehole while the well is being drilled. A bottom hole assembly at the lower end of casing includes a drill bit and an under reamer. The upper end of the casing is connected to a casing gripper, which in turn is connected to a top drive of a drilling rig. The casing gripper provides a seal for drilling fluid pumped down from the top drive into the interior of the casing. The casing gripper also supports the weight of the casing and transmits from the top drive torque to rotate the casing and the drill bit at the lower end of the casing. In some cases, the bottom hole assembly includes a mud motor that rotates the drill bit relative to the casing, the mud motor being driven by drilling mud pumped downwardly through it.
Since the string of casing extends upward through the conventional rig annular blowout preventer, closing the blowout preventer will only close the annulus surrounding the casing; it will not close the bore of the casing. On occasion, unexpected high pressure can be encountered, referred to as a “kick”, which would tend to push the drilling fluid up out of the casing. If the casing gripper and top drive are connected to the casing, the operator can control the kick by pumping fluid downward through the top drive and into the bore of the casing.
There are times, however, when the casing gripper and top drive will be detached from the casing. Of course, this occurs as each new joint of casing is picked up and secured to the upper end of the string of casing during the drilling operations. That interval is fairly short. More lengthy intervals occur, such as when rigging up to retrieve and/or rerun the bottom hole assembly. These operations are required if the pilot bit is worn and needs to be replaced prior to reaching total depth. Also, unless the bottom hole assembly is to be cemented in the well, it must be retrieved when the casing is at total depth and before cementing. Retrieval and re-running can be done a number of ways: with wireline; pumping the bottom assembly in and/or out; and tripping drill pipe into and out of the string of casing.
During the rig up and rig down time for retrieving a bottom hole assembly, it would be desirable to have an ability to control any unexpected pressure kick that might occur. Also, having a system of pressure control during the retrieval or rerun operation is important.